The tale of the tail

Sharks’ tails have always mystified biologists. Their relatives, hundreds of different species of fish, happily push themselves through the water with symmetrical tails that move from side to side. But most sharks are asymmetrical; the top part of their tails is larger than the bottom part, sometimes grossly so. George Lauder and Cheryl Wilga decided to look into this uneven enigma. Lauder is a professor of biology at Harvard University who has a life-long interest in the design of animals that live in water. He often works with Wilga, a biologist at the University of Rhode Island. The Navy helps support their research because the things they learn could lead to robotic submarines that move more like fish and less like robots. Using a complicated setup of laser light sheets that four dogfish swim through, high-speed video cameras, and sophisticated computer software, the researchers studied the flow of water over their asymmetric tails. Wilga and Lauder described this setup and their results in the Aug. 19, 2004, edition of the journal Nature. While a symmetrical fish tail leaves a one-part wake behind, the shark experiments clearly show a two-part wake. The larger upper lobe of a shark’s tail cuts the oncoming water slightly before the smaller lower lobe. This creates a wake within a wake, giving the shark both thrust and lift, both forward and upward motion.